Friction stir welding (FSW) is a relatively new welding process for joining together parts of materials such as metals, plastics, and other materials that will soften and commingle under applied frictional heat to become integrally connected. A detailed description of the FSW apparatus and process may be found in Patent Publications WO 93/10935; WO 95/26254; and U.S. Pat. No. 5,460,317, all of which are herein fully incorporated by reference. One useful apparatus for FSW is shown in FIGS. 1A and 1B. As shown, two parts, exemplified by plates 10A', and 10B' on a backing plate 12' are aligned so that edges of the plates to be welded together are held in direct contact. An FSW tool W' has a shoulder 14' at its distal end, and a nonconsumable welding probe 16' extending downward centrally from the shoulder. As the rotating tool W' is brought into contact with the interface between plates 10A' and 10B', the rotating probe 16' is forced into contact with the material of both plates, as shown. The rotation of the probe in the material and rubbing of the shoulder against the upper surface of the material produce a large amount of frictional heating of both the welding tool and the plate interface. This heat softens the material of the plates in the vicinity of the rotating probe and shoulder, causing commingling of material which, upon hardening, forms a weld. The tool is moved longitudinally along the interface between plates 10A' and 10B', thereby forming an elongate weld along the interface between the plates. The welding tool's shoulder 14' prevents softened material from the plates from escaping upward, and forces the material into the weld joint. When the weld is completed, the welding tool is retracted.
In order to produce a sound weld it is necessary to vary the length of the probe 16' according to a specific material thickness so that the probe extends substantially through the entire thickness of the material. This ensures that there is sufficient frictional heating to soften the plate material and that the material is welded through the full thickness of the plates 10A' and 10B'. When an FSW tool having a probe of fixed length is used for welding materials having a thickness outside the narrow range that the fixed length probe can effectively weld, incompletely consolidated weld joints with poor fracture strength and toughness are produced. Currently, to avoid producing poor welds, each tool is configured for a specific material thickness. Accordingly, different FSW tools, each with a specific probe length, are required for producing welds of different thicknesses. This significantly increases tool inventory costs as well as process costs associated with the time required to replace a tool suitable for producing a weld of one thickness with a tool suitable for producing a weld of a different thickness. A need exists for an FSW tool capable of producing welds of differing thicknesses.